They is a reason why directx 12 is following in Mantle footsteps and that's because it's simply the future of pc gaming..
It's not just about looking at frame rate, it's how the frame rate is delivered to the user the lower the latency the smoother the game is..
I laugh when I see someone say but dx11 got avg 80fps and mantle avg 70fps mantle sucks... Bah, bah, bah
Dx11 will never be better than Mantle, Mantle smashes it in everything from low frame rate latency to better use off the hardware/crossfire..
Having low level api like mantle or dx12 is massive it will also open the door for new technologies in gaming.
What I can't wait to see is how Mantle does when dx12 drops what happens to mantle and how is the performance between the two... If mantle dies then fine, least it's pushed the door for the better. And amd should be given thanks for doing so.